I think my favorite is still the drive-in, personally. With so much room to work with, it's possible to build some really massive structures (using the weapon drop exploit, natch) that you can get really creative.
That said, I still use Sanctuary as my home base because I'm all set up there and am too lazy to move. I will FT back to Home Plate occasionally for quick loot drops, but at the end of the day it all makes it back to Sanctuary.
The Drive in Theater got too crowded...plus, maybe because of my level or game progression, but I've had to almost completely wall it off and post heavy MGs all around because super mutants keep "visiting"
Also, you are aware that Spectacle Island has more space right?
Indeed. Although I haven't progressed far enough in game to see it in person yet. I hit a gamebreaking bug and had to quit playing. I just built a new PC last weekend, though, so maybe things will behave now.
Did the new PC fix it? And what was the bug (I has it on PS4, and reaaaalllly hoping Bethesda keeps to their word about having mod support on consoles).